In usual scenario new subscriber added to ORION database when you test connection (Add new ORION Source to AC).

And unsubscribe -- when you remove that record from AC's ORION Source list.

Unsubscribe problem (when you delete record from AC but don't clean up ORION database) can happen if:

- there was no connection between ORION-AC when you delete ORION source instance (network is down or ORION is down, etc.)

- you don't have that particular AC instance (so you can't unsubscribe).

If you have the same instance of AC that is subscribed to ORION you can try subscribe, unsubscribe and then subscribe again.

But if this instance is no longer valid (or ip/host changed, so AC claim to be different instance right now) than you must consult with ORION team how to do clean up properly (I unaware of all database relationships and mappings and what consequences can have direct clean up of Subscribers table).